"The term 'pure-blood' refers to a family or individual without Muggle (non-magic) blood. The concept is generally associated with Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry, whose aversion to teaching anybody of Muggle parentage eventually led to a breach with his three fellow founders, and his resignation from the school."

By 1995, the number of pure-blood families were declining. Most pure-blood families were related by then in an effort, by some, to retain their status by marrying only other pure-blood families, but it had led to inbreeding. While some families, such as the Malfoys, had allowed for marrying half-bloods, others such as the Gaunts, Lestranges, and Blacks, required their family to only marry pure-blood individuals, or those with no Muggle or Muggle-born parents or grandparents.[2][1] This refusal of marrying Muggles or Muggle-borns had lead to difficulty in propagating their families.[1]

Pure-blood individuals are people who have no Muggles or Muggle-borns as parents or grandparents.[4] Traditionalist pure-bloods like to keep the generations "pure" by breeding with other pure-bloods and are generally the ones who use the term 'Mudblood', a term considered derogatory by the wizarding world. Mudblood means Muggle-born, suggesting they have dirty blood and thus have no right to be a witch/wizard. Pure-bloods who do not share these prejudices are considered "blood traitors" by the ones that do.

Maintaining "purity"

"So-called pure-blood families maintain their alleged purity by disowning, banishing, or lying about Muggles or Muggle-borns on their family trees. Then they attempt to foist their hypocrisy upon the rest of us by asking us to ban works dealing with the truths they deny. There is not a witch or wizard in existence whose blood has not mingled with that of Muggles [...]"

Some pure-blood families can trace their pure-blood status through many generations of magical ancestors and deny ever having any Muggles within the family, such as the House of Black, the motto of which is "Toujours pur", meaning "Always (or Still) Pure".[6][7] However, the truth is that if they ever did exist in the past, true pure-blood wizards and witches do not exist today. They merely erase Squibs, Muggle-borns, and Muggles from their family trees.[1]

Half-bloods who consider blood purity very important also hide their Muggle ancestry, clinging to the magical heritage they do have.[8] Many Death Eaters are believed to have done this, and their leader himself took on the name Lord Voldemort out of a desire to not keep the name of his "filthy Muggle father".[9] It is likely that Voldemort told his followers he was a pure-blood, or that most of the Death Eaters knew better than to question Riddle's blood status, given the reaction of Bellatrix Lestrange to Harry's "accusation" that he was in fact a half-blood.[10]

Many pure-blood families, such as the Blacks and Gaunts, practise marrying cousins in order to maintain their pure-blood status, and disown any members who marry someone who is not a pure-blood.[7] This practice has been known to cause problems as families become inbred; for example, the Gaunts showed signs of violent tendencies, mental instability, and being enfeebled.[11][2] The number of pure-bloods is diminishing over time because of inbreeding.[1]

Most wizards and witches consider a person to be pure-blood if his or her parents and grandparents were not Muggles or Muggle-born.[4]

Attitude

"I really don't think they should let the other sort in, do you? They’re just not the same, they've never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, imagine. I think they should keep it in the old wizarding families."

Members of the pure-blood Weasley family: for sympathising with Muggles and being friendly with Muggle-borns, this pure-blood family is labelled as 'blood traitors'

Tom Riddle, later known as Lord Voldemort, continued his persecution of those who were not pure-bloods (ironically being a half-blood himself) with his followers, the Death Eaters. They strove to place pure-bloods in control of the wizarding world and to rid it of Muggle-borns. When Voldemort succeeded in taking over the Ministry of Magic during the height of the Second Wizarding War, his followers created the Muggle-Born Registration Commission in order to weed Muggle-borns out of the population and send them to Azkaban for allegedly stealing magic. Pure-bloods were also blatantly favoured within the Ministry and, to a lesser extent, Hogwarts School of Witchcraft and Wizardry, although half-blood was sufficient to avoid prosecution.[12]

However, there are pure-bloods who do not have these beliefs, and instead treat all magical people and Muggles as equals. These pure-blood folk are often labelled "blood traitors" by the more fanatical pure-bloods.[9]

Many characters mention throughout the series that blood purity doesn't matter, often using Hermione Granger in comparison to Ron Weasley and Neville Longbottom. Ron had struggled with living in the shadows of how much better students his older brothers were, and often would not try if he felt the situation was out of control, preferring to goof off or play with Chocolate Frog cards. Despite Neville's pure-blood heritage, he is not an extremely talented wizard, which made him a target for ridicule and difficulty, particularly from Professor Snape (in his later life he better grasped his full potential); however, Hermione is a supremely gifted witch despite her Muggle-born status, and performed very well right from the beginning.

The Harry Potter Glossary at Scholastic's official website lists the term "Trueblood" as "Someone born to pure magical parents." This term, however, is not known to have appeared in any official Harry Potter media. The nuance with "pureblood" may be that while the child of a half-blood and a pureblood counts as a pureblood, a trueblood can only be the child of two purebloods. That is however unconfirmed.

The concept of pure-blood is also similar to the real-life situation of nearly all European royal families being related, especially the British and (now extinct) German monarchies. Royals were only allowed to marry another member of the aristocracy, so most of the families were interrelated through marriage.

Author's comments

The expressions 'pure-blood', 'half-blood', and 'Muggle-born' have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his maternal grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted 'Aryan' or 'Jewish' blood...the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda.[4]

This statement indicates that all of a wizard or witch's grandparents must have magical heritage — i.e. that he or she cannot have a single Muggle or Muggle-born grandparent — in order for him or her to be considered pure-blood. The detailed and lengthy family trees maintained by pure-blood families such as the House of Black maintained a record of all the pure-bloods born and married into the family (as well documenting those who were blasted off to maintain the family's sense of purity).[10]

↑ 2.02.12.2Writing by J.K. Rowling: "The Malfoy Family" at Pottermore - "From the imposition of the Statute of Secrecy onwards, no Malfoy has married a Muggle or Muggle-born. The family has, however, eschewed the somewhat dangerous practise of inter-marrying within such a small pool of pure-bloods that they become enfeebled or unstable, unlike a small minority of fanatic families such as the Gaunts and Lestranges, and many a half-blood appears on the Malfoy family tree."

↑The Tales of Beedle the Bard - "Albus Dumbledore on “The Fountain of Fair Fortune”" - "So-called pure-blood families maintain their alleged purity by disowning, banishing or lying about Muggles or Muggle-borns on their family trees. They then attempt to foist their hypocrisy upon the rest of us by asking us to ban works dealing with the truths they deny. There is not a witch or wizard in existence whose blood has not mingled with that of Muggles, and I should there-fore consider it both illogical and immoral to remove works dealing with the subject from our students’ store of knowledge."

↑ 4.04.14.24.3FAQ at J. K. Rowling's Official Site - "The expressions 'pure-blood', 'half-blood' and 'Muggle-born' have been coined by people to whom these distinctions matter, and express their originators' prejudices. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as 'bad' as a Muggle. Therefore Harry would be considered only 'half' wizard, because of his mother's grandparents.
If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted 'Aryan' or 'Jewish' blood. I saw one in the Holocaust Museum in Washington when I had already devised the 'pure-blood', 'half-blood' and 'Muggle-born' definitions, and was chilled to see that the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda."

↑Harry Potter and the Half-Blood Prince, Chapter 12 (Silver and Opals) - "“The Death Eaters can’t all be pure-blood, there aren’t enough pure-blood wizards left,” said Hermione stubbornly. “I expect most of them are half-bloods pretending to be pure. It’s only Muggle-borns they hate, they’d be quite happy to let you and Ron join up.”"

↑ 10.010.1Harry Potter and the Order of the Phoenix, Chapter 35 (Beyond the Veil) - "“Shut your mouth!” Bellatrix shrieked. “You dare speak his name with your unworthy lips, you dare besmirch it with your half-blood’s tongue, you dare —” “Did you know he’s a half-blood too?” said Harry recklessly."

↑Writing by J.K. Rowling: "Draco Malfoy" at Pottermore - "In childhood, Draco associated mainly with the pure-blood children of his father’s ex-Death Eater cronies, and therefore arrived at Hogwarts with a small gang of friends already made, including Theodore Nott and Vincent Crabbe."

↑Writing by J.K. Rowling: "Draco Malfoy" at Pottermore - "In childhood, Draco associated mainly with the pure-blood children of his father’s ex-Death Eater cronies, and therefore arrived at Hogwarts with a small gang of friends already made, including Theodore Nott and Vincent Crabbe."

↑Harry Potter and the Deathly Hallows, Chapter 22 (The Deathly Hallows) - "The only place I’ve managed to find the name ‘Peverell’ is Nature’s Nobility: A Wizarding Genealogy. I borrowed it from Kreacher,” she explained as Ron raised his eyebrows. “It lists the pure-blood families that are now extinct in the male line. Apparently the Peverells were one of the earliest families to vanish.”"

↑Ch. 13 of Deathly Hallows confirms that the Selwyns are a pure-blood family, though it is unclear if Umbridge was being truthful about her own relation to them or her own blood status, as she was trying to "bolster her own pure-blood credentials". See Talk:Selwyn family for more information.

↑Slytherin’s only known descendants died with the Gaunts and Tom Riddle.

↑Bilius is a relative of the Weasleys, though it is unknown if he is arelation of Arthur of Molly.